The Federal Communications Commission and cable television testing organizations such as CableLabs have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard comprising for example, high definition television (HDTV) and standard definition (SDTV) digital signals for terrestrial broadcasting in 1995, and although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach.
The new ATV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonomously with "conventional" to represent conventional television in general.
In 1995 the FCC will test the so-called "Grand Alliance" system which is a proposed system being cooperatively developed by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992. This Grand Alliance system proposes to take the best features from those systems already tested in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has already decided on a coding algorithm which will comply with the source coding standards proposed by MPEG (Motion Pictures Experts Group). In addition, the RF transmission scheme selected by the Grand Alliance is the trellis coded 8VSB system designed and built by Zenith Electronics. Details of the Zenith VSB system are described in "Digital Spectrum Compatible--Technical Details", Sep. 23, 1991 and more recently modified and described in "VSB Transmission System: Technical Details", Feb. 18, 1994, which are incorporated by reference herein.
The technique used in the Zenith 8VSB modem to combat co-channel interference is as follows. A comb filter is used in the digital (e.g. HDTV) signal receiver to introduce nulls in the digital spectrum at the frequency locations of the conventional (e.g. NTSC) picture, color and the sound carriers. This provides a significant improvement in performance when conventional television, e.g. NTSC, is broadcast on a co-channel.
When co-channel interference from an NTSC signal is present at the HDTV receiver, the comb filter at the receiver is treated as a partial response channel in cascade with the trellis coder. An optimum decoder can then be developed which uses Viterbi decoding on an expanded trellis, the states of which correspond to the cascade of the states of the comb-filter and the trellis coder as described in "Principles of Digital Communication and Coding" authored by A. J. Viterbi and J. K. Omura and published by McGraw Hill in 1979, which is incorporated by reference herein.
For a comb-filter with a delay of 12 symbols, the number of trellis states are extremely large. To simplify their design, Zenith converts the MPEG coded and RS coded and interleaved data-stream from serial to parallel, then uses 12 parallel trellis encoders followed by a parallel to serial converter at the transmitter. The trellis decoder for the case when the comb filter is used, implements Viterbi decoding on a trellis with the number of states equal to two or four times the number of states of the trellis encoder. This is described in detail in "VSB Transmission System: Technical Details".
For the case when co-channel conventional television interference is absent, Viterbi decoding is implemented on a trellis with the number of states equal to the number of states of the trellis encoder. This is possible since pre-coding is not used in the transmitter.
The choice between the path afforded by simple trellis decoding or of that using the comb filter and the expanded trellis at the receiver is decided by the measured error-rate of the periodically sent data field sync symbols at the outputs of the post-comb filter and with no post-comb filter.
When both co-channel and AWGN (additive white Gaussian noise) are present however, the performance of the comb filter degrades dramatically. This is because the AWGN after the comb filter does not remain white, but gets "colored", in other words the noise samples are not independent of each other. This affects the performance of the trellis decoder which is optimized for performance in an AWGN channel. Since the co-channel conventional television interference is maximum at the fringe area where the signal power is small and hence the AWGN is large, this is indeed a scenario which must be taken into account. A first objective of the instant invention is therefore to improve the performance of an ATV receiver when co-channel interference and a high AWGN level are present.
The number of states of the trellis encoder is limited by the fact that the Viterbi decoder for the comb-filter path must operate on a trellis with at least double the number of states of the trellis encoder. This limits the AWGN performance of the trellis encoder/decoder when co-channel television interference is not present. A second object of the instant invention therefore is to improve the AWGN performance of the trellis encoder/decoder in an ATV receiver when co-channel television interference is not present.
The comb filter method of NTSC rejection requires the ATV spectrum to be shifted 45.8 khz with respect to the NTSC spectrum in order to align the nulls of the comb filter with the picture and color carriers as described in "VSB Transmission System: Technical Details". This causes the digital spectrum to spill over into the adjacent 6 Mhz channel which is undesirable for adjacent channel rejection. Another object of the invention is to do away with this frequency offset.
Finally, the switching between the use of a comb filter in the receiver or not, suggested by Zenith is cumbersome. A significant number of computations must be performed to determine whether the comb filter should be used or not. Furthermore the use of the comb filter also specifies the use of 12 parallel encoders and correspondingly 12 parallel decoders which also is cumbersome. Another object of the invention therefore is to avoid the use of a comb filter at the receiver.
In addition to other documents cited herein, this application incorporates by reference the following documents:
U.S. Pat. Nos. 5,086,340, 5,087,975 and 5,121,203 and U.S. Ser. Nos. 08/170,471, filed Dec. 20, 1993 and 08/197,773, filed Feb. 10, 1994.